MY CAR WON'T START NOW - REV COUNT DROPPED TO ZERO, NOW IT WON'T FIRE

1991 Mazda MX5

Help! My poor little car just died while driving home on the motorway last night! Rev count dropped to zero yet the car was technically still on - I turned it off, then tried to turn the car back on and it's turns over, but won't fire : (

I know next to nothing about cars - basically I'm trying to prepare myself for the worst for when I get it to a mechanic, so that when/if he tells me its going to cost an arm and a leg, I'm not going to embarrass us both by crying in his workshop! What could be the likely cause? (No point in trying to tell me how to fix it, haha! I wouldn't have the first clue!)

I filled her right up the day before - So before you ask me (like my insurance company did - argh) petrol is a-plenty!

Thanks and apologies in advance - being so clueless when it comes to cars is definitely not a good thing.